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UNIT 7

Ammonia Plant Control- A Detailed Example

This unit presents a detailed example of a control project for which costsand benefits can be calculated with reasonable certainty. The project, com-puter control of ammonia production, is described in narrative form withthe reader in the project developer's seat.

Learning Objectives When you have completed this unit you should:

A. Know why computer control of ammonia production is an attractiveproject.

B. Have a better understanding of the information required toestimate costs and benefits.

C. Appreciate the uncertainties that still exist even for a well-definedproject.

7-1. Benefit Identification

It is oil price crisis time (again). You are control engineer of a complex thatincludes a 1000 ton/day ammonia plant. Your plant uses the same tech-nology as 50 other plants around the country. A flow diagram is shown inFig. 7-1. Several of these plants have installed computer control and arereporting benefits. Should you put your plant under computer control?What loops should be controlled? What benefits will computer controlproduce? You decide to talk to some of the vendors and study the open lit-erature.

Several vendors are offering package systems with preprogrammed con-

trol loops. The literature indicates that almost all of the plants that haveinstalled computer control have controlled hydrogen/nitrogen ratio(H/N), steam/gas ratio (S/G), and synthesis loop pressure. All of theseloops offer cost savings. Optimal H/N and S/G ratios minimize theamount of synthesis gas that must be processed to produce a ton of ammo-nia. Maximizing loop pressure maximizes the conversion per passthrough the synthesis loop. Are these cost savings attractive?

7-2. Benefit Evaluation

Ammonia production uses natural gas as both raw material and fuel, sonatural gas is by far the largest cost component. Oil prices have already

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72 UNIT 7: Ammonia Plant Control A Detailed Example

Fig. 7-1. Kellogg Ammonia Process (Reprinted by Permission from Hydrogen Processing,November 1980, Copyright 1980 by Gulf Publishing Co. All rights reserved)

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UNIT 7: Ammonia Plant Control A Detailed Example 73

gone up this year, and gas prices are expected to follow. Efficiency is mea-sured by the amount of natural gas consumed per unit product, expressedas mscf/ton or, in metric units, m3/kg. Your plant is typical, with naturalgas usage of 37.5 mscf/ton (34020 m3/kg). Computer control has been

reported to produce efficiency increases of 1 to 5%. What can you expectfrom your plant?

The historic control performance of your plant can be estimated from logsheets. 95% ranges around average values are 0.13 for H/N, 0.05 forS/G, and 0.7 atmospheres for loop pressure. Consistent 95% ranges of0.05 for H/N, 0.02 for S/G, and 0.3 atmospheres for loop pressure arereported under computer control. Unfortunately, published data arescarce on control performance and efficiency before and after applicationof computer control. One unpublished report lists typical results. Historiccontrol performance of your plant is slightly worse than typical beforeresults.

One paper (Ref. 2) mentions a production increase of 1% when the H/Nratio standard deviation is reduced by 0.1 units. You expect to reduce the95% range from 0.26 to 0.1 units. Since the 95% range equals 4 standarddeviations, this is equivalent to a 0.04 unit standard deviation reduction.Assuming linearity, this should produce a 0.4% efficiency increase. Similarreasoning leads to an estimate of a 0.3% efficiency increase from better

S/G ratio control. The efficiency increase from better pressure control canbe estimated from Fig. 7-2. There is a constraint at 148 atmospheres, sooperation has been at an average pressure of 146.6 atmospheres to avoidviolating the constraint. This value is 4 standard deviations from the con-straint. With computer pressure control, the pressure set point can bemoved to 147.4 atmospheres while maintaining the relationship betweenaverage value and the constraint [see Eq. (4-1)]. This shift in average pres-sure will increase production, or efficiency, by 0.4%.

S/G and pressure control involve no new sensors and should be on line

whenever the computer is running. H/N ratio control depends on newcomposition measurements, including a gas chromatograph. Your planthas had difficulties with chromatographs, so you assume that this loopwill be out of service 20% of the time. This assumption reduces theexpected benefit from H/N control to 0.8 x 0.4% = 0.32%. The totalincrease in efficiency that you expect is 0.32% + 0.3% + 0.4% = 1.02%. Howmuch is this worth?

Your plant now buys incremental natural gas for a spot price of $2.50/mscf, and following the oil price increases, the price is expected to climb.

Without allowing for price escalation, your plant will use 37.5 x 1000 x$2.50 = $93,750 worth of natural gas per operating day. The plant runs 350days per year, so the expected 1.02% increase in efficiency should be

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worth $93,750 x 350 x 0.0102 = $335,000/year. Assuming that computercontrol will take six months to implement and that the system will have auseful life of six years, benefit cash flow will be as shown in Fig. 7-3.

7-3. Cost Evaluation

Costs of this project are relatively easy to estimate, since vendors are will-ing to offer fixed-price turnkey packages that include application soft-ware. The best price quote you receive is for $200,000. System installationcost will be only about $20,000, since the computer will be installed in anexisting control room and most of the signals are already available in thecontrol room.

Fig. 7-2. Effect of Pressure on Ammonia Production (Adapted from Ref. 1 by permission from

Hydrocarbon Processing, No. 1980, Gulf Publishing Co., all rights reserved.)

Fig. 7-3. Benefit Cash Flows

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Additional equipment costs will include $10,000 for an uninterruptiblepower supply and $30,000 for additional instrumentation. Installationcosts for these items will be $20,000. In-house engineering costs are esti-mated at $20,000, most of which is for electrical design and start-up assis-

tance. Operator training is included in the vendor's turnkey price.

Operating costs include system and instrumentation maintenance. Youhave no in-house computer maintenance capability, so you contract withthe vendor to provide hardware and software support for $10,000/year. Inaddition, you budget $5,000/year for instrument maintenance, principallyfor the H/N loop sensors. Cost cash flows are shown in Fig. 7-4. It isassumed that the system and instrumentation are purchased at the start ofthe project, and other first costs are incurred during the first year.

7-4. Project Evaluation

Overall cash flows are shown in Fig. 7-5. Your company's guideline forenergy-saving projects like this one is an internal pre-tax rate of return of40% (The oil crisis has produced double-digit inflation, so the cost of capi-tal is high). This project qualifies easily, with an IRR of 87.5%.

It should be realized that many features of this project are unusually risk-free. The control strategy and equipment are already in use at several sim-ilar plants, so the chance of technical success is high. Costs are virtuallycertain. Application software, usually the hardest category to estimate, iscovered by a turnkey fixed-price contract. Benefits come from raw mate-rial and energy savings and therefore are not strongly dependent uponmarket conditions. Internal rate of return more than doubles the guideline

value, so small changes in costs or benefits will not affect project viability.

Fig. 7-4. Cost Cash Flow

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7-5. Epilogue

The author has gone through this exercise for two ammonia plants. Com-puter control systems were approved and installed for both plants. Con-trol performance was similar to that reported for other plants (Ref. 3).Process improvement was larger than anticipated, as average efficiencyimproved by 1.75%. Internal rate of return was only slightly higher thanestimated, since natural gas spot prices, contrary to all expectations,

declined to as low as $1.25/mscf.

The reader probably realizes that some things have changed since theevents described in this chapter. A new control scheme for a petrochemi-cal plant is likely to involve model predictive control, manipulating multi-ple variables to hold operations close to limiting constraints. Software willaccount for a higher percentage of costs. Raw material and energy costsare much higher. None of these changes affects the basic themes of thischapter. Upgrading control of a plant where feedstock and energy are themajor costs is still likely to be an attractive project when these costs

increase sharply. Risk will be low if the control scheme has already beenapplied to similar plants. Volatility of natural gas prices will quite possi-

Fig. 7-5. Overall Cash Flows

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bly affect estimated benefits - the current (late 2005) price of $13/mscf isnot chiseled in stone.

References

1. Blevins, T. L., and Langley, K., 1980 Process Control Models.Hydrocarbon Processing, 59, 11, pp. 197-201.

2. Daigre, L. C., and Nieman, G. R., 1974. Computer Control ofAmmonia plants. Chemical Engineering Progress, 70, 2, pp. 50-53.

3. Friedmann, P. G., 1978. Evaluating Computer Control ofAmmonia Plants. AIChE Ammonia Plant Safety Symposium, 20,pp. 85-88.

Exercises

7-1. How would the internal rate of return be affected if the plant produced only300 tons/day?

7-2. List some possible scenarios that would make this project a loser.

7-3. What would the annual benefits be if production is limited by feedstockavailability and increased production can be sold for $120/ton? Assume

that incremental production expenses other than natural gas are $10/tonand that total natural gas consumption is unchanged.

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